The present invention relates to a magnetic fluid seal apparatus, and particularly to a magnetic fluid seal adapted to insure that the area where the driving shaft of a hard disc drive passes through the wall of the casing is kept airtight.
The hard disc in a computer is a magnetic recording element contained within a casing and driven to rotate by means of a motor provided outside the casing. A hard disc should never be fouled by dust or other contamination, so the area where the drive shaft of the motor penetrates the wall of the casing must be provided with a seal apparatus that prevents the passage of dust. To this end, magnetic fluid seal apparatuses are commonly employed as described in many prior patent documents such as JP-A-62-110080 (the term "JP-A" as used herein indicates an unexamined published Japanese patent application), JP-A-U-58-191423 (the term "JP-A-U" as used herein indicates an unexamined published Japanese utility model application), JP-A-U-61-13025, JP-A-U-61-44067, JP-A-U-61-79070, JP-A-U-62-195261, JP-A-U-61-204027, JP-A-U-63-8419, JP-A-U-63-139325, JP-B-U-63-29944 (the term "JP-B-U" as used herein indicates an examined Japanese utility model publication) as well as U.S. Pat. No. 4,628,384 and 4,692,826.
While the details of the magnetic fluid seal apparatuses described in these prior publications differ in various ways, basically, they have the construction shown in FIG. 15. At 1 in FIG. 15 is a housing made of a nonmagnetic material such as aluminum or a synthetic resin which is typically secured to the wall of the casing of the hard disc. At 2 is a shaft made of a magnetic material such as iron. The magnetic fluid seal apparatus generally indicated at 3 is positioned in the cylindrical space 6 between the inner surface 4 of the housing 1 and the outer surface 5 of the shaft 2.
The magnetic fluid seal apparatus 3 comprises an annular permanent magnet 7 that is magnetized in the axial direction (in the horizontal direction as viewed in FIG. 15) and which is sandwiched between a pair of annular pole pieces 8 and 9 made of a magnetic material, with a magnetic fluid 10 being retained in the gap 11 between the inner edge of each pole piece and the outer surface 5 of the shaft 2 by the magnetism of the permanent magnet 7. The outside diameter of the permanent magnet 7 and each of the pole pieces 8 and 9 is set to be equal to or slightly greater than the inside diameter of the housing 1, so that the magnetic fluid seal apparatus 3 composed of the members 7, 8, 9 and 10 is fitted in the housing 1 and secured to its inner surface.
By this arrangement, the magnetic fluid seal apparatus 3 is positioned between the inner surface 4 of the housing 1 and the outer surface 5 of the shaft 2, so irrespective of the rotation of the shaft 2 within the housing 1, the magnetic fluid 10 held between the outer surface 5 of the shaft 2 and the inner edge of each of the pole pieces 8 and 9 insures a complete seal between the inner surface 4 of the housing 1 and the outer surface 5 of the shaft 2. It should be noted here that the use of two pole pieces 8 and 9 is not a mandatory requirement and only one pole piece suffices for the magnetic fluid seal apparatus as long as a bearing assembly or some other means positioned in the vicinity of the magnetic fluid seal apparatus 3 aids in forming a magnetic field.
If the magnetic fluid 10 spreads over the surface of the shaft 2 (or, in other configurations, over the lateral sides of pole pieces 8 and 9 or the inner surface of the housing 1), the chance of its splashing into the clean area where the hard disc is accommodated will increase. If the magnetic fluid 10 is deposited on the surface of the hard disc, the computer using this hard disc can operate erroneously. With a view to solving this problem, JP-B-60-48668 proposes that oil-repelling films 12 typically made of polytetrafluoroethylene (PTFE) be formed in the neighborhood of the gaps 11 where the magnetic fluid 10 is retained as shown in FIG. 16, thereby preventing it from splashing into the clean area through the gaps 11. The similar proposal of the above-mentioned proposal is also shown in the JP-B-58-27426. In order to produce a high-performance, durable and reliable magnetic fluid seal apparatus at low cost, it is necessary that a film which is as thin as possible, that is tenacious and that has a strong oil repelling ability be easily and efficiently formed. However, none of these conditions are fully satisfied by the technique described in JP-B-60-48668. First, not only is it costly to form films 12 from an oil-repelling material such as PTFE, but also the films so formed become so thick that the dimensions of the gaps 11 are apt to change and may fail to retain a sealing amount of the magnetic fluid 10. Second, if the PTFE films 12 separate from the surface on which they were formed, comparatively large dust particles will become suspended in the space where the hard disc is accommodated and this can be another cause of erroneous operation of the computer using said hard disc.